1. Field of the Invention
This invention relates to sulfur recovery plants of the type used to recover sulfur from petroleum refinery gas streams containing hydrogen sulfide. Refinery feedstocks are increasingly of the high sulfur type, and as environmental considerations require production of low sulfur fuels and fuel oils, the need for recovery of hydrogen sulfide from petroleum refinery gas streams has increased.
The sulfur recovery process most suited for refinery acid gas cleanup is the vapor phase or modified Claus process. Of the processes capable of handling hydrogen sulfide rich, hydrocarbon laden acid gas, the modified Claus process is the most reliable and the easiest to operate. In some cases, such as where stringent pollution codes are in effect, the addition of a Shell Claus off-gas treating (SCOT) process is generally employed. The SCOT process for cleanup of tail gas from a Claus unit is popular because it uses equipment and technology with which refinery operating personnel are familiar.
2. Prior Art
The primary objective in starting up a sulfur recovery plant is to get the unit on stream in minimum time without exceeding acceptable pollution levels. Exposure of the environment to hydrogen sulfide and sulfur dioxide is greatest during start-up. The first step of the start-up process is the plant heat-up. Heat-up is conventionally carried out by burning fuel gas with injected air in the reaction furnace and in in-line burners if they exist. Excess air is required to completely burn the fuel gas to carbon dioxide. Without excess air, carbon is deposited in the plant equipment. However, excess air can react with residual sulfur, carbon and iron pyrites generating dangerously high temperatures. Sulfur dioxide formed from burning sulfur is a pollutant, and sulfur trioxide (formed with high excess air and high temperature) is corrosive with water. Generally, it is preferred to use a low amount of excess air and to take the resulting carbon lay-down.
Heat-up procedures utilizing inert gas are described in U.S. Pat. Nos. 3,366,455 and 3,617,221. However, these processes do not eliminate oxygen, and the inert gas is merely used as a heat sink to lower the temperature of the products of combustion. U.S. Pat. No. 3,719,744 describes a process including flushing a sulfur recovery unit with steam to remove condensed sulfur prior to shut-down of the unit.
Prior to this invention, there has been no process available for heating up a sulfur recovery unit which did not involve the introduction of oxygen into the unit.